A crystal structure of graphite is shown in FIG. 1. The structure can be properly described using the space group of P63/mmc, and its layer-structured characteristic evidenced from the depiction in FIG. 1 and conventionally known. Graphite is constructed through honeycomb-structured, carbon layers held by Van der Waals forces. Since Van der Waals forces comprise one of the weakest chemical bonding in materials, the graphite layers may be theoretically exfoliated from the graphite surface as “graphene” layers without much sacrifice or consumption of energy and work. Nonetheless, graphite has been reported as a good electrical conducting material especially in the in-plane direction (e.g., the ab plane shown in FIG. 1, 55 to 65 μΩ cm in resistivity) owing to the overlapping of the P orbital (ρ bonding) of carbon atoms. Such features make graphene a desirable top-layer for substrate material surfaces if the electrical conductivity is important for material performance for certain applications.
In view of materials for lithium ion battery applications, electrical conductivity and lithium ion mobility largely determine the performance (e.g., rate capability or C-rate) of the material. Conventionally, carbon black (e.g. acetylene black) is used for the enhancement of electrical conductivity of the electrode containing battery active materials through mechanisms, such as addition, in the step of slurry formation during electrode processing. However, such addition mechanisms possess limited enhancement of electrode conductivity for one or a plurality of reasons. For instance, one reason may be the lack of contact between the material and the carbon black owing to the presence of binder. Another reason may be that the carbon black cannot penetrate to any place of the material, especially when material particles are small or mesoporous. Further, the high surface area nature of the carbon black makes the slurry formation procedure difficult during electrode processing.
Various conventional techniques include the incorporation of graphene with metal oxides in solution with the presence of surfactant. Indeed, this approach is somewhat similar to the techniques described above except the carbon source has been changed from carbon black to graphene. One problem with these conventional processes is that high surface area graphene layers do not adequately bond to the substrate material (e.g., metal oxide) in the solution. As a result, the adherence of graphene to the substrate material is questionable, and “free graphene” may not form proper bonding to the substrate material in the later stage (e.g. after drying or with subsequent heat treatment) or can be even present in the final product. Furthermore, the graphene in the solution may curve in the end (during drying or even in the solution) and form nanotubes that cannot form proper bonding to the substrate materials. Overall, the outcome is about the same, which is a drastic increase in the surface area of the as-synthesized material. Moreover, the performance of the as-synthesized material is strongly dependent on the weight percentage of “graphene” being present in the material.